Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 342

There is reluctance by some repair product manufacturers to adequately address
the problem, with a very limited range of appearances available from their
materials in different countries. There is a propensity for specifiers to recommend
overcoating of the whole building, such as with a high-build acrylic paint in order
to achieve a uniform appearance, which is also easily matched in future if further
repairs become necessary.
As a general rule, these traditional repair systems have a number of disad-
vantageous aspects such as different thermal expansion coefficient compared to
concrete, weak bonding, disappearance, environmental and health hazards, and
even costly. Therefore, many researchers proposed biodeposition as an alternative
and eco-friendly technique to enhance the durability of building materials and
structures. So far, most of studies have focused on improvement in the durability
of concrete or mortar surface treatment by biodeposition (Reddy, et al. 2012 ). De
Muynck et al. ( 2008a , b ) compared the durability (concerning capillary water
uptake and gas permeability) of concrete when its surface was treated with pure
and mixed cultures of ureolytic bacteria. They concluded that the type of bacterial
culture and the medium composition had a profound impact on CaCO 3 crystal
morphology, being that the use of pure cultures resulted in a more pronounced
decrease in the uptake of water. They also concluded that the durability perfor-
mance obtained with cultures of the species B. sphaericus was comparable to the
ones obtained with conventional water repellents (silanes, siloxanes). De Muynck
et al. ( 2008a , b ) studied different durability parameters (carbonation, chloride
penetration, and freezing and thawing) confirming that the biodeposition treatment
showed a similar protection toward degradation processes when compared to some
of the conventional surface treatments under investigation. They also mention the
need for investigations regarding the durability of the treatment under acidic
media. They further mentioned that biological generated calcite is less soluble than
the one inorganically precipitated, thus suggesting a higher performance. Achal
et al. ( 2011a , b , c ) mention a six times reduction in water absorption due to the
microbial calcite deposition. In a different study, Achal used a phenotypic mutant
of S. pasteurii (BpM-3) with improved urease activity also reporting a significant
reduction in water absorption, permeability, and chloride permeability. Okwadha
and Li ( 2011 ) used bacterium S. pasteurii strain ATCC 11859 to create a bio-
sealant on a PCB-contaminated concrete surface reporting a reduction on water
permeability by 1-5 orders of magnitude. They also state that the treated concrete
had a high resistance to carbonation. Li and Qu ( 2012 ) confirm that bacterially
mediated carbonate precipitation on concrete surface reduces capillary water
uptake, leading to the carbonation rate constant to be decreased by 25-40 %. Bio-
materials can lead concrete to a more sustainable in civil engineering. Much
research has drawn the attention in the field of concrete surface treatments by
biodeposition; however, it is still far from being a proved and reliable technique
capable of replacing current common concrete surface treatments based on organic
polymers sealers. Biodeposition helps to fill micropores and cracks, thus reducing
its permeability. However, the highly alkaline pH of concrete reduces the activities
of the bacteria. To overcome this problem, different authors have suggested the use